Spot morphology of non-contact printed protein molecules on non-porous substrates with a range of hydrophobicities

Non-contact inkjet printing technology is one of the most promising tools for producing microarrays. The quality of the microarray depends on the type of the substrate used for printing biomolecules. Various porous and non-porous substrates have been used in the past, but due to low production cost and easy availability, non-porous substrates like glass and plastic are preferred over porous substrates. On these non-porous substrates, obtaining spot uniformity and a high signal to noise ratio is a big challenge. In our research work, we have modified pristine glass slides using various silanes to produce a range of hydrophobic glass substrates. The hydrophobicities of the slides expressed in the contact angle (theta) of a sessile drop of water were 49 degrees, 61 degrees, 75 degrees, 88 degrees and 103 degrees. Using a non-contact inkjet printer, microarrays of biotinylated biomolecules (BSA and IgG) were produced on these modified glass substrates, pristine (untreated) glass and also on HTA polystyrene slides. The uniformity of the spots, reflecting the distribution of the biomolecules in the spots, was analyzed and compared using confocal laser scanning microscopy (CLSM). The quality of the spots was superior on the glass slide with a contact angle of similar to 75 degrees. We also investigated the influence of the hydrophobicity of the substrate on a two-step, real diagnostic antibody assay. This nucleic acid microarray immunoassay (NAMIA) for the detection of Staphylococcus aureus showed that on highly hydrophilic (theta <10 degrees) and hydrophobic substrates (theta > 100 degrees) the assay signal was low, whereas an excellent signal was obtained on the substrates with intermediate contact angles, theta similar to 61 degrees and theta similar to 75 degrees, respectively.